Heating element for canned foods



United States Patent Olilice 3,126,225 Patented Feb. 4, 1964 3,126,225 HSJA'HNG ELEMENT FR SANNED FGBS Rnsseii Bliss Akin, Wilmington, Bel., assigner to E. l. du Pont de Nemours and Company, Wiirningion, Del., a corporation of Belau/are Filed Mar. 16, 1196i, Ser. No. 96,3l S Claims. ('Cl. 126-2162) This invention relates to a novel means for heating canned foods, and, more particularly, it relates to a shaped polymeric material capable of heating canned foods.

Campers, soldiers, and others who live outdoors have always had the problem of providing themselves with hot food without also burdening themselves with cooking equipment. Military forces have heated canned food by the use of tablets of trioxane as fuel. This fuel has many advantages such as low luminosity and slow rate of burning, but it has the same disadvantage as larly natural fuel in that the canned -ood must be heated through the bottom of the can, thus frequently causing scorching of the food at the bottom of the can.

It is an object of this invention to provide a novel fuel element for heating canned food. It is another object of this invention to provide a slow-burning fuel element which can be used to heat the contents of a can of food Without the danger of scorching the food. It is another object of this invention to provide a novel method of attaching fuel to a can. Still other objects will appear from the detailed explanation of the invention which follows:

rEhe vforegoing objects are accomplished by providing a shaped structure of polyoxymethylene, the structure being such that it fits closely around the lower portion of the outside surface of the can. This structure may be in the form of a springy helical coil of an elongated strand, or it might be a cup-shaped cover to be placed around the can, or it might be a springlike cylindrical sheet to be snapped into place around the can. `In the preferred embodiment of this invention the structure comprises a mediurn to high molecular weight polyoxymethylene with or without additives to provide or enhance certain features. A flamepropagating means may be applied to the lower extremities of said structure so as to provide a quick and even ignition of the structure on all sides of the can.

A more complete understanding of this invention may be had by an inspection of the attached drawings. FIG- URE 1 shows a can l with the fuel element in the `form of a helical coil 2 wrapped around the lower portion of the outside of the can. The cross-section of the strand forming coil 2 may he solid or hollow and may be round, rectangular, triangular, or any other convenient shape. ln FIGURE 2 the can 1 has a fuel element in the form of a springlike sheet 3 which can be spread open far enough to be slipped over the can and then released so that by spring action the sheet will close tightly around the can. in FIGURE 3 the can 1 is placed into a fuel element 4 in the shape of a cup which fits `closely to the outside contour of the can. If desirable, the upper edge of the cup it may be formed into a series of lingers or scallops 5 which are directed inwardly so as to apply a slight pressure against the can and thereby provide the frictional engagement to hold cup 4 in place. If will be understood that in any of these shapes, some of the combustible element may extend under the bottom of the can. In FlG- URE 4 the can 1 has a fuel element 7 which is vin the form of a helical coil of a tape shown in this embodiment to have a generally rectangular cross section.

Although there are several plastic materials Which can be burned and which can be formed into the shapes of these fuel elements, polyoxymethylene is especially desirable for this purpose. It burns slowly, with an imperceptible, blue darne, and without producing any ash, smoke, or soot. Polyoxymethylene is physically strong, unaffected by moisture, and can be fabricated into durable shapes which are flexible, tough, and springlike.

The burning characteristics of polyoxymethylene are such that Vif the coil 2, the sheet 3, or the cup 4 of the attached drawings were ignited at one location around the circumference of the can d, the llame from the burning polyoxy-rnethylene would not rapidly distribute itself completely around the circumference. AS a consequence, uneven heating of the contents of the can would result. Therefore, it is preferable that some flame-propagating means be provided around the lower extremities of the fuel element. This means is shown at 6 on each of FIG- URES 1, 2, 3, and 4. Several alternative means may be employed. The polyoxymethylene may be roughened or cut into small slivers or shavings to make it easier to ignite in the same fashion as kindling wood is prepared. A coating, which is much easier to ignite than polyoxymethylene, for example, nitrocellulose lacquer, may be applied to the lower portion of the fuel element. Still another alternative which has been fourid to be highly desirable is to apply a coating which `consists of 3 parts by weight of nitrocellulose lacquer and 1 part by Weight of feathery, fluffy particles of polyoxymethylene. A typical nitrocellulose lacquer might have the following composition:

Grams Dry nitrocellulose 18 Ethanol 1-1 N-butanol 10.7 Toluene 9.8 Xylene 17.3 Methyl ethyl fketone 16.6 Butyl acetate 16.6

Total Other flame-propagating means can be employed with equal effectiveness. The only requirement is that the flame be spread completely and rapidly around the can so that it may then cause the polyoxymethylene on all sides of the `can to burn at about the same time. -lf a self-igniting fuel element is desired, the head or a match may be axed to the Jdame-propagating means so that the fuel element can be put to use by merely striking that match.

The amount of polyoxymethylene in the fuel element will depend upon the thermal characteristics of the material to be heated as Well as the characteristics of the polyoxymethylene itself. 1n use as la fuel element it can be considered that about 700() Btu. of heat is theoretically available per pound of polyoxymethylene lburned. In actual tests it has been found that about 10-30% of the available heat is actually transferred to the contents of the can. For most types of `canned foods a fuel element containing from about 25-50 grams of polyoxymethylene per pound of food or per pint of liquid may be employed to heat the contents of the can from a temperature of about 35-50 F. up to about 15G-200 F.

The fuel element should iit the sides of the can as closely as possible so that there is no insulating layer of air between the fuel element and the can. This will help to insure the maximum elhciency for utilizing heat.

There are `several reasons why polyoxymethylene is particularly desirable for use in the fuel element of this invention. Polyoxymethylene melts at about 340-350 F. lFor the purposes of heating food Without scorching it this lis a desirable temperature to apply to the outside of the can. The flame of the burn'mg polyoxymethylene is, of course, at a much higher temperature, but it is insulated from the can by a layer of molten polyoxymethylene.

The polyoxymethylene must be abricable into the shapes desired for these fuel elements. Generally, this means that the polyoxymethylene must have a number average molecular weight of at least 10,0%. The higher has been a disadvantage of the previously used low molecular weight polymers of oxymethylene, such as trioxane. The following examples illustrate the use of the fuel element of this invention.

Example 1 High molecular Weight plastic polyoxymethylene was extruded in the form of a tubing 5716 inch in diameter u with a wall thickness of 26 mils. The tubing was wrapped in the form of a helical coil over a mandrel 2% inches in diameter and annealed in an oil bath rat 320 F. for 20 minutes. After removal of the coil from the mandrel it relaxed to form a coil with an inside diameter or about 21/2 inches. The coil was then `cut to an axial ilength of about 21/2 inches. Two turns at one end .of the coil were coated with a 4nitrocellulose lacquer containing about 25% by weight of eathery particles of polyoxymethylene, Vfollowed by two coats of unmodified nitrocellulose lacquer. The lacquer was dried in anair oven at 70 F. for one hour.'

A coil prepared as described above, containing about 33 grams of polyoxymethylene, was wrapped around a can of beef Y stew (contents of the 'can weighing 190 grams). The fuel element was ignited with the stew being at a temperature of 35F. In 7-8 minutes in a moderate breeze the temperature of the stew was raised to 150 F. and there was no evidence of scorching of the stew.

Example 2 Time to Maximum Fuel Complete Tempera- Test No. Weight, Combusture Grams tion, Min. Reazhed,

`It is not intended that this invention be limited to fuel elements composed entirely of polyoxymethylene. Many combustible matenials are granular or are frangible,v or are otherwise not suitable for preparation into structural shapes. These materials might, however, be mixed with a resinous polyoxymethylcne and the resulting mixture could be fabricated into structural shapes. Thus, the heating element lof this invention may comprise pollyoxymethylene mixed with any of a wide variety of materials which may aid in the combustion of the element; for example, combustible carbonaceous materials, oxidizing agents, and intumescing agents. it may be desirable in `sorne instmces to provide a high surface area, for example, by employing a cellular, or foamed, polyoxymethylene; or alternatively by employing a ribbed o-r porous form of polyoxymethylene.

The yforegoing description illustrates several embodi- -rnents of this invention, Vbut it is not intended that the invention be limited in any fashion other than as described in the appended claims.

There are several 'related materials of construction which are equivalent in this invention because they consist primarily of recurring loxymethylene (*CH2O) units. ene herein. The polyoxymethylene intended to be emplcyed in this invention may be a homopolymer of formaldehyde or of triox-ane or it may be a copolymer, blend, or mixture of these polymeric substances with other materials. lt is important only that 'the polyoxymethylene employed lhave (l) sulicient strength to be fabricated into stable shapes and to have the spring-like resiliency needed to provide frictional engagement between the heat- Y ing element and the can, and (2) a chemical structure such that the combustion characteristics Vare substantially the same as those of the homopolymer of formaldehyde.

I claim:

1l. The combination of a can of lfood and a fuel element adapted `for use vin heating said can, said fuel eleme'nt being composed of a high molecular weight, thermoplastic, fabrica-ble polyoxymethylene and being vin the eform of a hollow cylindrical article, the wall of which has the natural springlike resiliency of said polyoxymethylene md which is of such a size as to t snugly around said can which is to be heated, the exact shape and form of said wall being such as to permit said article toV be manually radially expanded suiciently to be placed around said can, and, upon release, to radially contact and to grip said can with suicient pressure to remain in position 'around said can without other means of attachment.

2. The combination of claim l in which said hollow cylindrical article is a helical coil of a tube.

3. The combination of claim l in which the Vsaid' hollow cylindrical article is a helical coil of a tape.

4. The combination of claim l in which the said hollow cylindrical article is a cup-shaped cover 'fitting over Y the lower portion of said can.

5. The combination of claim l in which the said hole Switzerland Sept. V2, 1946 These materials are all labelled polyoxymethyl-- 

1. THE COMBINATIONOF A CAN OF FOOD AND A FUEL ELEMENT ADAPTED FOR USE IN HEATING SAID CAN, SAID FUEL ELEMENT BEING COMPOSED OF A HIGH MOLECULAR WEIGHT, THERMOPLASTIC, FABRICABLE POLYOXYMETHYLENE AND BEING IN THE FORM OF A HOLLOW CYLINDRICAL ARTICLE, THE WALL OF WHICH HAS THE NATURAL SPRINGLIKE RESILIENCY OF SID POLYOXYMETHYLENE AND WHICH IS OF SUCH A SIZE AS TO FIT SNUGLY AROUND SAID CAN WHICH IS TO BE HEATED, THE EXACT SHAPE AND FORM OF SAID WALL BEING SUCH AS TO PERMIT SAID ARTICLE TO BE MANUALLY RADIALLY EXPANDED SUFFICIENTLY TO BE PLACED AROUND SAID 